Printed circuit board including EMI reducing circuits, an information processing apparatus having the board and a method to select the circuits

ABSTRACT

A printed circuit board (PCB) has a plurality of electromagnetic interference (EMI) reducing circuits, which reduce electromagnetic waves emitted from the PCB, and a plurality of switching devices, such as MOS transistors, relays and DIP switches, to enable and disable the EMI reducing circuits. The EMI reducing circuits connected between border portions of a voltage and a ground layers in the PCB include at least a capacitor. A combination of the EMI reducing circuits which gives a minimum amount of electromagnetic waves emitted from an apparatus including the PCB is selected as a suitable combination of EMI reducing circuits. An information processing apparatus having the printed circuit boards has means for selecting a combination of the EMI reducing circuits which allow the minimum EMI emitted for the apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a printed circuit board having means fordecreasing electromagnetic radiation emitted therefrom, an informationprocessing apparatus having such printed circuit board(s), and a methodfor adjusting a suitable combination of the circuits arrangement toreduce the electromagnetic radiation.

More particularly this invention relates to decreasing electromagneticradiation from mobile terminals, such as mobile computers and notebookcomputers, having a function of data communication with other facilitieswith a wireless interface.

2. Description of the Related Art

In recent years, higher performances of the apparatus need higher clockfrequencies to operate CPU and electronic components in the apparatus.And the higher clock frequencies cause higher speed variations ofvoltage pulses between a voltage layer and a ground layer in printedcircuit boards (PCBs) in the apparatus; and, therefore, electromagneticradiation is radiated from the boards.

On the other hand, the development in computer downsizing has resultedin so called mobile computers, notebook computers, and laptop computers,which are easily carried. And these small sized computers can alsocommunicate with other facilities with wireless interfaces. As theseapparatus with wireless interface are particularly susceptible to theelectromagnetic radiation emitted therefrom, it is very important toreduce the electromagnetic radiation.

As the electromagnetic radiation maybe cause erroneous operations inother nearby facilities, there are usually provided metallic shieldingscovering the apparatus and containment arrangements on the PCBs toreduce the radiation. To more successful development in the apparatus,it is preferable to reduce the electromagnetic radiation from PCBsitself, because it is possible to reduce the electromagnetic radiationwithout increasing the metallic shielding structures, which adverselyaffect the computer downsizing.

One of the containment arrangements has been proposed in Japanese PatentApplication No. 08-073987 (Unexamined Patent Publication No. 09-266361),where a PCB structure includes capacitors connected between a voltagelayer and a ground layer at the border portion of the PCB. As thecapacitors act as by-pass capacitors, the electromagnetic radiationcaused from the voltage oscillation is reduced. In spite of preventingthe electromagnetic radiation without affecting the scale of PCB's size,the prior art has traditional drawbacks described hereinafter, as thecapacitance values and locations of the by-pass capacitors are fixed.That is, even in the prior art it must take labor-intensive practice toadjust in a short time the characteristics of circuits for the reducingelectromagnetic radiation, according to the rapid progress of computers,such as adoption of higher operating frequency in CPU. After redesignsand trial productions, values of capacitors and their locations on PCBsare decided.

A conventional design method to prevent the electromagnetic radiation isa manner of a cut-and-try one. After a number of trials, metallicshielding structures or characteristics of electronic components such ascapacitors used in the circuits preventing electromagnetic radiation aredecided. This conventional design method takes long time troublesomeworking to decide specifications of parts and components, and it isnecessary to redesign the circuits to reduce the electromagneticradiation in almost every case of changing a design of apparatus orother circuit configuration.

As undesired electromagnetic radiation from electro-equipment result inelectromagnetic interference (“EMI”), a circuit for reducing theelectromagnetic radiation is referred to hereinafter designate as an“EMI reducing circuit.”

SUMMARY OF THE INVENTION

An object of this invention is to provide a PCB having an improvedarrangement of EMI reducing circuits each of which includes a circuitelement for decreasing electromagnetic radiation form the PCB and aswitching device selectively connecting the circuit element to a portionof a printed circuit pattern for enabling and disabling the circuitelement. In preferred embodiment for the object of the presentinvention, the PCB has a plurality of EMI reducing circuits comprising abypassing circuit or damping circuit, and a switching device. Theswitching device is connected to the bypassing circuit or the dampingcircuit, and is for enabling and disabling the bypassing circuit or thedamping circuit.

Further object of this invention is to provide an information processingapparatus having a function minimized electromagnetic radiation. Inpreferred embodiment for the further object of the present invention,the apparatus has the above mentioned PCB and further comprises a memorydevice for storing information of a plurality of predetermindedcombination of the EMI reducing circuits to be enabled and disabled, anda control unit for transmitting signals to the EMI reducing circuits foropening or closing the switching devices corresponding to theinformation stored in the memory device.

Still another object of this invention is to provide a mobile terminalhaving the PCB(s) and a detachable wireless communication module. Inpreferred embodiment for the still another object of the presentinvention, the mobile terminal further includes the memory device, thecontrol unit, a data storing device for storing original data, acomparing unit for obtaining a difference between the original data anda data received by the communication means by comparing these data, anda means for selecting a minimum difference out of a plurality of thedifferences and transmitting a data of the combination of the EMIreducing circuits resulting in the minimum difference to the controlunit.

Still another object of this invention is to provide a method foradjusting a suitable combination of the circuit characteristics in amobile terminal used for data communication. In preferred embodiment forthe still another object of the present invention, the method foradjusting the EMI reducing circuits mounted on a printed circuit boardcomprises a step of composing a plurality of combination of the EMIreducing circuits to be enabled or disabled, a step of performinginformation processing in enabling the EMI reducing circuits in each ofthe combination, a step of measuring electromagnetic radiation emittedfrom the printed circuit board in performing the information processing,a step of comparing amounts of the electromagnetic radiation, a step ofselecting a minimum amount out of the amounts, and a step of enablingthe EMI reducing circuits of the selected combination resulting in theminimum amount.

Other objects and advantages of the present invention will be apparentfrom the following description, the appending claims and accompanyingdrawings.

A BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are schematic diagrams of the present invention;

FIG. 2A and FIG. 2B schematically illustrate a first preferredembodiment of a printed circuit board in accordance with the presentinvention; and FIG. 2A is a top plane view of the board; and FIG. 2B isan enlarged section taken along an arrow line A—A of FIG. 2A;

FIG. 3 schematically illustrates a second preferred embodiment of aprinted circuit board in accordance with the present invention;

FIG. 4 schematically illustrates a third preferred embodiment and showsa diagram of an EMI reducing circuit device mounted on the printedcircuit board;

FIG. 5 schematically illustrates a fourth preferred embodiment and showsa diagram of an EMI reducing circuit on the printed circuit board;

FIG. 6 schematically illustrates a fifth preferred embodiment and showsa diagram of an EMI reducing circuit mounted on the printed circuitboard;

FIG. 7 schematically illustrates a sixth preferred embodiment and showsa diagram of an EMI reducing circuit mounted on the printed circuitboard;

FIG. 8 schematically illustrates a seventh preferred embodiment andshows a diagram of an EMI reducing circuit mounted on the printedcircuit board;

FIG. 9 and FIG. 10 schematically illustrates a eighth preferredembodiment; and FIG. 9 shows a fragmentary detail of an EMI reducingcircuit mounted on the printed circuit board; and FIG. 10 schematicallyillustrates a diagram of enlarged section taken along an arrow line D—Din FIG. 9;

FIG. 11A and FIG. 11B schematically illustrate a ninth preferredembodiment of a printed circuit board in accordance with the presentinvention; and FIG. 11A is a top plane view of the board and FIG. 11B isan enlarged section taken along an arrow line B—B of FIG. 11A;

FIG. 12 schematically illustrates a tenth preferred embodiment and showsa diagram of EMI reducing circuits;

FIG. 13 is a diagram of combination of control signals and circuitconstants in the tenth preferred embodiment;

FIG. 14 schematically illustrates a diagram of an eleventh preferredembodiment of the present invention applied to a single signal line on aprinted circuit board;

FIG. 15 schematically illustrates a twelfth preferred embodiment of anapparatus in accordance with the present invention;

FIG. 16 schematically illustrates a thirteenth preferred embodiment ofan apparatus in accordance with the present invention;

FIG. 17 is a schematic flow chart to select a suitable combination ofthe EMI reducing circuits in an apparatus shown in FIG. 16;

FIG. 18 schematically illustrates a fourteenth preferred embodiment ofan apparatus in accordance with the present invention; and

FIG. 19 schematically illustrates a fifteenth preferred embodiment of anapparatus in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The principle of the present invention is described hereinafterreferring to FIG. 1 where two types of EMI reducing circuit are shown.One of the types of the circuit includes a bypass circuit and aswitching device as shown in FIG. 1A; and another of the types of thecircuit includes a damping circuit and a switching device as shown inFIG. 1B. Referring to FIG. 1A showing a partly enlarged illustration ofa multilayer printed circuit board 1, the multilayer printed circuitboard 1 has a voltage layer 2 and a ground layer 3 on both surfaces of ainsulating layer 4. An arrangement formed with these three layers issimilar to an arrangement of a microstrip antenna for transmitting andreceiving electromagnetic waves. To reduce the electromagnetic radiationemitted from this multilayer arrangement, a first EMI reducing circuit 5comprises a capacitor C1 and a switching device SW1. A second EMIreducing circuit 6 comprises a capacitor C2, a resistor R2 and aswitching device SW2. These EMI reducing circuits 5 and 6 are enabledand disabled by the switching devices SW1 and SW2, where to enable thecircuits 5 and 6 means to electrically connect these circuits 5 and 6between the voltage layer 2 and the ground layer 3; and to disable thecircuits 5 and 6 means to disconnect these circuits 5 and 6 from thevoltage layer 2 or the ground layer 3. The first and second circuits 5and 6 are preferably connected to each border portion of the voltagelayer 2 and the ground layer 3. In being enabled, each of the EMIreducing circuits 5 and 6 acts as a circuit including bypass capacitorC1 and C2 respectively, and a voltage oscillation between the voltagelayer 2 and the ground layer 3 is reduced. Consequently, theelectromagnetic radiation is reduced.

It is preferable to connect a resistance element in series with acapacitor, such as R2 shown in FIG. 1A, because the resistance elementconsumes energy caused by a current flowing.

Referring to FIG. 1B showing another simplified arrangement of thepresent invention applied to a signal line 7, EMI reducing circuit 10and 11 have resistors R3 and R4 wired in series with switching devicesSW3 and SW4 respectively, where the resistors R3 and R4 are selectivelyconnected to circuits 8 and 9 by these switching devices SW3 and SW4respectively. As abrupt variations in signals transmitted from thecircuit 8 are eased by R3 and/or R4 connected by the switching devicesSW4 and SW5; the electromagnetic radiation emitted from the signal line7 is reduced.

The above mentioned prior art has a disadvantage in that the value ofcapacitors provided between the both layers 2 and 3 are constant and notchangeable for the suitable value to minimize the EMI if specificationsof other electronic components mounted on the PCB are modified.

On the contrary, the present invention provides a PCB which includes theEMI reducing circuits having suitable circuit constants, which areselected by the switching devices, such as SW1, SW2, SW3 and SW4. Theseswitching devices are electrically connected in series with the bypasscapacitors C1 and C2, and the damping resistors R1 and R2 respectively.Therefore, in the present invention the switching devices make itpossible for the EMI reducing circuits to have suitable circuitconstants if specifications of other electronic components mounted onthe PCB are modified.

For example, in the arrangement shown in FIG. 1A the capacitance isselected out of C1, C2, C1+C2, and in FIG. 1B the resistance is selectedout of R3, R4, R3R4/(R3+R4).

Referring to FIG. 2 to FIG. 14, preferred embodiments of EMI reducingcircuits are fully shown in accordance with the present invention.Preferred embodiments shown in FIG. 2 to FIG. 13 are related to thebypass-type EMI reducing circuit; and a preferred embodiment shown inFIG. 14 is related to a damping-type EMI reducing circuit.

FIG. 2 is a first preferred embodiment of the present invention and FIG.2A schematically illustrates a top plane view of a PCB 20. FIG. 2B showsa detail of connections of a EMI reducing circuit 26 arranged in a EMIreducing unit 27 to a voltage layer 22 and to a ground layer 23. The EMIreducing circuit 26 is shown hereinafter as preferred embodiments.

In FIG. 2B, the PCB 20 is a multilayer printed-circuit board composed ofthree insulating layers 21, the voltage layer 22, the ground layer 23,and conductive patterns 24 and 25 as outmost layers for mounting circuitcomponents and forming signal lines. The voltage layer 22 and the groundlayer 23 are composed with an electrical conductive plane respectively.The present invention is applicable to other PCB which has an othernumber of layers.

In FIG. 2A, the circuit components mounted on the PCB 20 are not shownto make the first preferred embodiment clear except ones relateddirectly to the invention.

A plurality of EMI reducing units 27 are mounted along the boarder ofthe PCB 20, where each of the EMI reducing circuits 26 is included inthe EMI reducing unit 27. These EMI reducing units 27 are connected to acontrol circuit unit 28 with conductive patterns 24. An upper controlunit (not shown in FIG. 2) transmits selection-signals to the controlcircuit unit 28. The control circuit unit 28 transmits control signalsin accordance with the selection-signals to each of the EMI reducingcircuits 26 through conductive patterns 24. Then the switching devicesin the EMI reducing circuits 26 are operated to be ON or OFF by thecontrol signals.

The EMI reducing units 27 has three terminals; a first terminal 30 forreceiving the control signal to operate the switching device in the EMIreducing circuit 26, a second terminal 32 and a third terminal 33 forconnecting the circuit 26 between the voltage layer 22 and the groundlayer 23. The first terminal 30 is connected to the conductive pattern24, the second terminal 32 is connected via a through-hole 31 to thevoltage layer 22, and the third terminal 34 is connected via athrough-hole 33 to the ground layer 23.

While the EMI reducing circuit 26 in the EMI reducing unit 27 areelectrically connected to the PCB 20 as above-mentioned via theterminals 30, 32, and 34 in the first preferred embodiment shown in FIG.2B, the EMI reducing unit 27 may be electrically connected with leadwires or other arrangement suitable to the surface mounting instead ofthe manner of the first preferred embodiment. Still more, the EMIreducing unit 27 may be connected via a connector mounted on the PCB 20to the PCB 20.

And still more, while the conductive patterns 24 and 25 are arranged onthe outmost layers in the first embodiment, the conductive pattern 24 or25 may be arranged on the intermediate layers.

FIG. 3 schematically shows a top view of a PCB 40 as a second preferredembodiment of the present invention for shortening a length of theconductive pattern 42 or for forming an area to mount other circuitcomponents (not shown). In FIG. 3, the conductive pattern 42 and controlcircuit units 41 are similar to the conductive pattern 24 and thecircuit control unit 28 in the first preferred embodiment shown in FIG.2A respectively. The control circuit unit 41 may be divided into anumber of smaller units and be located separately as shown in FIG. 3,where the control circuit unit 41 is composed with two smaller units andlocated so as to form a central area to mount other circuit components.

FIG. 4 schematically shows a third preferred embodiment of the presentinvention, where a single of EMI reducing circuit 50 is included in theEMI reducing unit 27. In the circuit 50, a MOS transistor TR5 is used asa switching device, a capacitor CS as a capacitive element, and aresistor R5 as a resistance element which converts a bypass current toheat.

A gate G of the MOS transistor TR5 is connected to the first terminal30; a drain D to the second terminal 32; and a source S to the capacitorC5; and the resistor R5 to the third terminal 34, respectively.

Although the MOS transistor having high input impedance suitable to theEMI reducing circuit is preferable as switching device, bipolartransistors may be used as the switching devices.

In the third preferred embodiment shown in FIG. 4, as the MOS transistoris N-channel type, an electrical path between the drain D and the sourceS turns on when a high level control signal is input into the gate G.And then the EMI reducing circuit 50 results in being electricalconnection between the voltage layer 22 and the ground layer 23 shown inFIG. 2B. That is, the EMI reducing circuit 50 is enabled, and the highfrequency current, which causes EMI, flows from the voltage layer 22 tothe ground layer 23. Therefore the EMI is reduced.

While the EMI reducing circuit 50 includes a pair of the capacitor C5and the resistor R5 as shown in FIG. 4, the circuit 50 may comprise theMOS transistor TR5 and the capacitor C5. And the values of the capacitorC5 or the resistor R5 are not necessary to be equal in every EMIreducing circuit. A plurality of the EMI reducing units 27 is mounted onthe border of the PCBs as shown in FIG. 2 and FIG. 3.

Still more, an essential part of an EMI reducing circuit 51 is shown inFIG. 5 as a fourth preferred embodiment of the present invention. InFIG. 5, the same elements are designated by similar numerals in FIG. 4.In the present embodiment, a two-state type relay RL is used as theswitching device so as to open and close an electrical path in responseto the control signals and to sustain the electrical path till the nextcontrol signal.

When the control signal from the control circuit unit (not shown) isinput via the terminal 30 to the COIL of the relay RL at a state of apath S0-S1 being connected, the state is converted from S0-S1 connectedto S0-S2 connected. That is, the EMI reducing circuit 51 is convertedfrom an enabling state (S0-S1 connected) to a disabling state (S0-S2connected). Adversely the state of the path S0-S2 being connected turnsto the state of S0-S1 connected by inputting the control signal. Usingthe relay RL having the above-mentioned performance, a current tosuspend the state of electrical path is unnecessary and effective toreduce a power consumption of the apparatus. As shown in FIG. 1A, theEMI reducing circuit 51 may comprise the relay RL and the capacitor C6without the resistor R6.

Further still more, an essential part of EMI reducing circuits 52 and 53is shown in FIG. 6 as a fifth preferred embodiment of the presentinvention. In FIG. 6, the same elements are designated by similarnumerals in FIG. 4. In the fifth preferred embodiment, two EMI reducingcircuits 52 and 53 are included in the EMI reducing unit 27. The EMIreducing circuit 52 comprises a MOS transistor TR7, a capacitor C7, andthe resistor R7. The EMI reducing circuit 53 comprises a MOS transistorTR8, a capacitor C8, and the resistor R7. The resistor R7 is used incommon to the both circuits 52 and 53 for miniaturizing the EMI reducingunit 27.

A gate G of the MOS transistor TR7 is connected to the first terminal30; a drain D to the second terminal 32; and a source S to the capacitorC7; and the resistor R7 to the third terminal 34, respectively. In thesame fashion shown in the EMI reducing circuit 53, a gate G of the MOStransistor TR8 is connected to the first terminal 30′; a drain D to thesecond terminal 32; and a source S to the capacitor C7; and the resistorR7 to the third terminal 34, respectively.

In the fifth preferred embodiment, although two conductive patterns 24and 24′ to transmit each of the control signals to control the MOStransistors TR7 and TR8 are necessary, this arrangement enables toselect an EMI reducing circuit from four circuits which may havedifferent circuit constants respectively. That is, the arrangementallows a more suitable selection of the EMI reducing circuit to reduceEMI. As shown in FIG. 1A, the EMI reducing circuits 52 and 53 maycomprise the MOS transistors TR7 and TR8, and the capacitors C7 and C8without the resistor 7.

An essential part of an EMI reducing circuit 60 is shown in FIG. 7 as asixth preferred embodiment of the present invention. In the sixthembodiment, a MOS transistor TR9 is mounted in the control circuit unit28 to miniaturizing a size of the EMI reducing unit 27. In FIG. 7, adrain D of the MOS transistor TR9 is connected to a terminal 61 of thecontrol circuit unit 28; and the terminal 61 and the terminal 32 areelectrically connected with a conductive pattern 63 formed on the PCB20. In the same fashion above-mentioned, a source S of the MOStransistor TR9 is connected to a terminal 62; and the terminal 62 andthe terminal 30 are electrically connected with the conductive pattern24. And a capacitor C9 and a resistor R9 are arranged in series andconnected between the terminals 30 and 34. A gate G of the MOStransistor TR9 is connected a circuit component (not shown) in thecontrol unit 28. As shown in FIG. 1A, the EMI reducing circuit 60 maycomprise the MOS transistor TR9 and the capacitor C9 without theresistor R9. An advantage of the sixth preferred embodiment is in thatmore number of the EMI reducing units 27 can be mounted on the border ofthe PCB 20, where electromagnetic radiation are easily emitted.

An essential part of seventh preferred embodiment of the presentinvention is shown in FIG. 8; and a manually operative switching deviceis mounted for diminishing a control circuit unit and increasing an areato mount other circuit components. In the present embodiment, four EMIreducing circuits 55, 56, 57, and 58 are arranged in the EMI reducingunit 27. A DIP switch having four switches SW10, SW11, SW12, and SW13,each of which operates independently, is mounted in each of the EMIreducing circuits 55, 56, 57, and 58. These switches SW10, SW11, SW12,and SW13 are connected to each of capacitors C10, C11, C12, and C13respectively. A resistor R10 is used as a common resistance element ofthe EMI reducing circuits 55 and 56; and a resistor R12 is used as acommon resistance element of the EMI reducing circuits 57 and 58.Instead of the DIP switch, it is possible to use other DIP switcheshaving different number of switch or other mechanical switches, such astoggle switches. And it is preferable to use a small mechanical switchhaving many electrical contacts. As shown in FIG. 1A, the EMI reducingcircuits may comprise DIP switch and the capacitors without theresistors.

An eighth preferred embodiment of the present invention is describedhereinafter, using FIG. 9 and FIG. 10. In the eighth preferredembodiment, electroconductive resilient clips 71 are used as theswitching devices. FIG. 9 shows an essential border portion of a PCB 70of typical arrangement of the embodiment, where two EMI reducingcircuits including the capacitors C14 and C15, the resistors R14 andR15, and the two resilient clips 71 are shown. FIG. 10 shows across-sectional cut view taken along line D—D looking in the directionof the appended arrows of FIG. 9. In FIG. 9 and FIG. 10, the capacitorsC14 is connected via a conductive pattern 72 to the resistor R14 and theresistor R14 is connected to a ground layer 78 via a conductive pattern74 (shown in FIG. 9) with a through-hole (not shown). And also anotherterminal of the capacitor C14 is connected via another through-hole 76(shown in FIG. 10) to a conductive pattern 75.

The resilient clip 71 electrically connects these conductive patterns 75and 77, which are formed on opposite outermost surfaces of a PCB 70respectively. The conductive pattern 77 is electrically connected to avoltage layer 73 via a through-hole 79.

As described above, the resilient clip 71 connects the EMI reducingcircuits to the voltage layer 73 and the ground layer 78: the EMIreducing circuit is enabled. On the contrary, when the resilient clip 71is removed, the EMI reducing circuit is disabled. As the resilient clips71 are removable from the PCB 70, the resilient clips 71 are used as theswitching devices in the present embodiment.

Notches 80 which are U-shaped cut at edges of the PCB 70, as shown inFIG. 9, prevent the resilient clips 71 from carelessly being removed.The resilient clips 71 being easily produced have effects to lower thecost of the apparatus and electrical power consumption. As shown in FIG.1A, the EMI reducing circuits of the eighth preferred embodiment maycomprise the capacitors and the resilient clips without the resistors.

The ninth preferred embodiment of the present invention is schematicallyshown in FIG. 11A and FIG. 11B. Recently several kinds of power supplyvoltages are employed in a single PCB; therefore, several voltage layersare formed in the PCB. The ninth preferred embodiment provides a PCBwhere a plurality of EMI reducing circuits are connected to borderportions of each conductive pattern of voltage layers.

In FIG. 11A shows a top view of a PCB 90 mounting a plurality of EMIreducing circuits 27 and 27′. The PCB 90 has a first voltage layer 91and a second voltage layer 92. Both of the layers 91 and 92 areindicated with doted lines and are arranged in a position in thedirection of thickness of the PCB 90. A plurality of the EMI reducingunits 27 are mounted on and connected to the border portion of the firstvoltage layer 91; and a plurality of the EMI reducing units 27′ aremounted on and connected to the border portion of the second voltagelayer 92 hereinafter.

A control circuit unit, conductive patterns from the control circuitunit to the EMI reducing units 27 and 27′, and other circuit componentsmounted on the PCB 90 are not shown in FIG. 11A for the sake ofsimplicity. FIG. 11B is an enlarged fragmentary sectional view takenalong an arrow line B—B shown in FIG. 11A.

In FIG. 11B, although the PCB 90 in the ninth preferred embodiment hasthree insulating layers designated by the numeral 93, the presentinvention may be applicable to a PCB comprising an other number oflayers.

A manner of electrical connection of the EMI reducing circuits 26 and26′ is like the connection in the first embodiment shown in FIG. 2B; andthe connection is described hereinafter. The terminal 30 of the EMIreducing unit 27 is connected to a conductive pattern 94 so as toreceive control signals from the control circuit unit (not shown). Theterminal 32 is connected via a through-hole 95 to the first voltagelayer 91. A terminal 34 is connected via a through-hole 97 to a groundlayer 96. As the same manner of the above-mentioned connection of theEMI reducing unit 27, a terminal 30′ of the EMI reducing unit 27′ isconnected to a conductive pattern 98; a terminal 32′ is connected via atrough-hole 99 to the second voltage layer 92; a terminal 34′ isconnected via a through-hole 100 to the ground layer 96.

As the EMI reducing units 27 and 27′ are mounted alongside each boarderportion of the first voltage layer 91 and the second voltage layer 92respectively, the EMI emitted from the PCB 90 having a plurality of thevoltage layers 91 and 92 is reduced too.

Though the PCB 90 has explanatorily two voltage layers 91 and 92, thepresent invention is applicable to PCB(s) having more than two voltagelayers; and still more the present invention is applicable to a PCBhaving more than a single ground layer as the almost same fashion in theembodiment described above. Even still more, as the manner of connectingEMI reducing units via through-holes between voltage layers and a groundlayer is described in the ninth embodiment, the present invention isapplicable to a PCB having voltage layers or ground layers composed indifferent vertical locations along thickness of the PCB.

A tenth preferred embodiment of the present invention is describedhereinafter. The tenth preferred embodiment concerns increasing a numberof EMI reducing circuits mounted on a border portion of PCB(s). Asdescribed above, the gap between the voltage layer and the ground layeris one of sources from which the EMI is radiated; electrical connectionsof EMI reducing circuits to a border portion of the voltage layer andthe ground layer result in effective reduction of the EMI. And stillmore, for reducing the EMI, it is effective to select a combination ofsuitable circuits out of ones having different circuit constant.Therefore, the EMI reducing circuits mounted in an EMI reducing unitpreferably may have not equal circuit constant but different ones ordifferent ones synthesized from equal circuit constants. The EMIreducing units composed as described above will effectively reduce EMIeven if an adopted CPU has different operating frequency or higher clocksignal.

The tenth preferred embodiment provides a PCB for the above purpose andis described in detail hereinafter referring to FIG. 12 and FIG. 13.

FIG. 12 shows a schematic diagram of an essential part of the tenthpreferred embodiment. The EMI reducing circuits 110, 111, 112, 113, 114,and 115 are composed of six N channel MOS transistors, TR16, TR17, TR18,TR19, TR20, and TR21 and three diodes, D16, D18, and D20 respectively.Each of the MOS transistors TR16, . . . TR20, and TR21 performs bothfunctions of a switching device and a resistor. The backward-biaseddiodes D16, D18, and D20 as shown in FIG. 12, perform function of acapacitor caused by capacitance between an anode and a cathode of eachdiodes D16, D18 and D20 respectively. The MOS transistor TRIG16, . . .TR20, and TR21 make circuit conductive when a high level signal is inputto each of gates Gs of the MOS transistors; and the resistances betweeneach of sources Ss and drains Ds perform resistors consuming a currentflowing form a voltage to a ground layer. Each drain D of the MOStransistors TRiG, TR20, and TR21 are connected to the voltage layer,each source S to the ground layer, and each gate G to a serial/parallelconverter 116.

Hereinafter is described how to select the EMI reducing circuits 110,111, 112, 113, 114, and 115 to be connected between the voltage layerand the ground layer.

A selection signal S transmitted from an upper unit (not shown) is inputto a selector 118 in a control circuit unit 120. Receiving the selectionsignal S, the selector 118 reads out data, for example (1 0 0 0 0 0),from a RAM 119, which stores a plural data of combination indicatingwhich EMI reducing circuit is enabled or disabled.

The data (1 0 0 0 0 0) is transmitted to the serial/parallel converter116 in a serial data form. The reason why the data (1 0 0 0 0 0) istransmitted in serial data is to reduce the number of signal lines fromthe control circuit unit 120 to the serial/parallel converter 116, whichis preferably mounted nearby the EMI reducing circuits, and to allowarea for mounting other circuit components. The serial data (1 0 0 0 00) is converted into the parallel data (1,0,0,0,0,0) in theserial/parallel converter 116. Each datum is accordingly transmitted tothe each gate G as a control signal. In this example, the datum “1”means a high level signal, which makes the switching device close, thatis, the EMI reducing circuit enable.

In FIG. 13, are shown the six combinations of the control signals P1,P2, P3, P4, P5, P6 and the circuit constants, that is, resistance andcapacitance. For the simplicity, the control signals transmitted fromthe serial/parallel converter 116, and combinations of resistance andcapacitance are shown out of the theoretical sixty-four combinations. Asshown in FIG. 13, in the case of the combination (P1, P2, P3, - - - ,P6) being (1, 0, 0, 0, 0, 0), the high level signal “1” is input to onlythe gate of MOS transistor TR16 and the path between the source S andthe drain D of the MOS transistor TR16 is closed (conductive). Thereforethe EMI reducing circuit has circuit constants of resistance R16 andcapacitance C16, which shunt the voltage layer and the ground layer.

In the arrangement of the tenth preferred embodiment, the EMI reducingcircuit is capable of being selected one of sixty four circuitconstants; that is, it is possible to select the EMI reducing circuitsuitable to reduce EMI out of the circuit having many different circuitconstants.

And even more, as the MOS transistors TR16, TR17, TR18, TR19, TR20, andTR21 act as the switching devices, the EMI reducing circuits and theunit are possibly arranged small.

Referring now to FIG. 14, there is schematically shown an eleventhpreferred embodiment of the present invention. The eleventh preferredembodiment is different from the embodiments described heretofore; andprovides an EMI reducing circuit to reduce EMI emitted form a signalline transmitting high frequency signals.

As shown in FIG. 14, EMI reducing circuits 133 and 134 are connectedbetween a clock generator 130 and a CPU 131.

Although the eleventh preferred embodiment may be applied to any signalline through which clock signals CLn, CLn−1, . . . , CL2 are transmittedin the present embodiment shown in FIG. 14, the EMI reducing circuits133 and 134 are applied to only a single signal line 132 transmitting acloak signal CL1.

The EMI reducing circuits 133 and 134 having MOS transistors TR22 andTR23 respectively are connected in series with the signal line 132through drains Ds and sources Ss of the MOS transistor TR22 and TR23respectively. The gates Gs of the MOS transistor TR22 and TR23 areconnected to an unshown control circuit unit and receive the controlsignals IN1 and IN2 respectively. In the eleventh preferred embodiment,the MOS transistors TR22 and TR23 are N channel-type MOS transistors andthe resistance values of between drain D and source S are r22 and r23respectively. The MOS transistors TR22 and TR23 open or close the signalline 132 by a high or a low-level input signal into each gate G, and theresistance between drain D and source S acts as resistance for damping ahigh frequency signal in the signal line 132. The resistance values areselected according to the level of the control signal IN1 and IN2. Inthe eleventh preferred embodiment, the MOS transistors TR22 and TR23perform both of the switching devices and the resistors.

While the embodiment shown in FIG. 14 is arranged with the two MOStransistors TR22 and TR23, bipolar transistors, relays, or mechanicalswitches, such as DIP switches and toggle switches, are also availableas the switching devices. Carbon film resistors, metal film resistors,or thick film resistors are also available as resistors for damping.

In the third, the fifth, the sixth, the tenth, and the eleventhpreferred embodiments, although the EMI reducing circuit comprises theMOS transistor as the switching device, a bipolar transistor may beapplicable to the switching device.

From the descriptions in the first to the eleventh embodiments, it willbe understood that the present invention has an advantage to enable theselection of the suitable combination of reducing circuits to reduce EMIeven in case of changing circuit components mounted on PCBs.

And still more, the present invention has advantages to lower the costof the apparatus and PCBs and to shorten a period of designing them,because of unnecessity of redesign and trial production to decidesuitable circuit specifications in EMI reducing circuits.

In the description above, the electric switching devices such as the MOStransistors and relay are preferable, because of enabling a fasterselection of a most suitable combination of EMI reducing circuits.

On the other hand, as mechanical switches described in the seventh andthe eighth embodiments do not need the control circuit unit andconductive patterns to transmit control signals, the area on the PCBsfor the unit and the patterns is allowed for other circuit componentsand the cost of the apparatus is lowered.

A twelfth embodiment of the present invention is shown in FIG. 15. FIG.15 shows an information processing apparatus 170 having a plurality offirst EMI reducing circuits 176 and a plurality of second EMI reducingcircuits 183 thereon described above so as to reduce EMI emitted fromthe information processing apparatus 170. In FIG. 15, elements necessaryto explain the essence of the present invention are shown, but typicalelements of an information processing apparatus, such as a main memory,a disk unit, pointing devices are not shown.

In FIG. 15, the information processing apparatus 170 has a main PCB 171,a display unit 178, a keyboard 179, and an extended PCB 181. The firstEMI reducing circuits 176 are mounted on the main PCB 171; and thesecond EMI reducing circuits 183 are mounted on the extended PCB 181respectively.

On the main PCB 171 are mounted a CPU 172, a ROM 173, which stores acombination data of the first EMI reducing circuits 176 and the secondEMI reducing circuit 183, a first control circuit unit 175 forcontrolling the first EMI reducing circuits 176, an I/O control unit 174for controlling the display 178 and the keyboard 179, and a RAM 180storing a plurality of measurements. And on the extended PCB 181electrically connected through an interface unit 177 to Bus on the PCB171 are mounted a plurality of EMI reducing circuits 183 and a secondcontrol circuit unit 182. In the information processing unit 170arranged as shown in FIG. 15, the way of selecting the first EMIreducing circuit 176 and the second EMI reducing circuit 183 will bedescribed hereinafter.

The data of combination of the first and the second EMI reducingcircuits 176 and 183 to be enabled are stored in the ROM 173. A startcommand is input from the keyboard 179. At next step, the CPU 172 readsout a first combination data from a plurality of combination data storedin the ROM 173 in accordance with a predetermined sequence, and sendsthe combination data through bus lines to the first and the secondcontrol circuit units 175 and 182 respectively. The first and the secondcontrol circuit units 175 and 182 turn on the switching devices in theEMI reducing circuits 176 and 183 in accordance with the combinationdata respectively.

In this instance, on the display unit 178 the combination data or signsinstead of the combination data, such as ID numbers, are displayed.

In the next step, the CPU 172 processes a program, which ispredetermined for this EMI measurement, such as a computation of theratio of the circumference of a circle to its diameter. During thecomputation, the measurement instruments (not shown) measure the EMIemitted from the apparatus 170. After the computation, the measuredvalues are input with the keyboard 179, displayed on the display unit178, and stored in the RAM 180. In this manner, the values correspondingto every stored combination data are taken by the measurement, displayedon the display unit 178, and stored in RAM 180.

After each measured values corresponding to all combination data isstored in RAM 180, the CPU 172 searches a minimum value in the measuredvalues. And a combination data corresponding to the minimum value issent to the first and the second control circuit unit 175 and 182respectively; accordingly the corresponding a plurality of first andsecond EMI reducing circuits 176 and 183 are enabled respectively.

As instruments, circumstances and conditions of measurement of EMIemitted from an apparatus are well known, the descriptions related tothese are not described.

In the twelfth preferred embodiment, though the measured values aremanually inputted to the apparatus 170 through the keyboard 179, it ispossible to automatically transmit the measured values from themeasurement instruments via a cable to the apparatus 170 in order toshorten a time to determine the first and the second EMI reducingcircuits 176 and 183 to be enabled.

As there is the possibility that the amount of EMI is depend on aprogram processed by the CPU 172, it is preferable to process a sameprogram in every measurement.

As the twelfth preferred embodiment provides the information processingapparatus including PCBs mounting EMI reducing circuits thereon, whichare selectively enabled, the apparatus is able to control the amount ofEMI emitted from the apparatus itself.

A thirteenth preferred embodiment of the present invention isschematically shown in FIG. 16 and FIG. 17. FIG. 16 shows a blockdiagram of an information processing apparatus of the thirteenthpreferred embodiment and FIG. 17 shows a flow chart for selecting themost suitable combination of EMI reducing circuits in the informationprocessing apparatus. The thirteenth preferred embodiment provides aninformation processing apparatus 220 having a wireless interface and aPCB with EMI reducing circuits 223.

FIG. 16 shows elements necessary for an understanding of the thirteenthpreferred embodiment. A display unit 236 and a keyboard 237 areconnected to the apparatus 220 as I/O devices. In the apparatus 220, aninformation-processing unit 221 comprises the PCB, on which EMI reducingcircuits 223 and a control circuit unit 222 are mounted.

In case of transmitting the data from the information processingapparatus 220 to other facilities, the data processed in theinformation-processing unit 221 is sent to a modulator 224 where thedata is mixed with carrier. And the modulated data is sent to a RF(Radio Frequency) transmitter 225, and then sent to an antenna 227through a switch 226 and radiated, where the switch 226 is turned to “S”as shown in FIG. 16. Conversely in case of receiving data from otherfacilities, the data received by the antenna 227 is input to a RFreceiver 228 through the switch 226, where the switch 226 is turned to“R”.

By the RF receiver 228 and a demodulator 229, the data is detected,amplified and converted into signals in a digital form, and then inputinto the information processing unit 221. In this manner the informationprocessing apparatus 220 is capable of transmitting and receiving datato/from other facilities.

In the apparatus 220, a manner of adjusting EMI reducing circuits 223,which may have equal or different circuit constants, to be enabled isdescribed hereinafter in detail.

The essential part is a comparing unit 230, which compares the receiveddata with an original data, which is described in detail hereinafter.That is, the comparison between a received data and the original data isperformed at every predetermined combination of the EMI reducingcircuits 223 to be enabled at a same time. By the comparison, thecombination which gives minimum bit error rate is selected as apreferable one.

Now the original data is explained. Generally in data transmission, dataare composed with a predetermined format. And some parts in the formatare assigned to specific data which have specific number of digitlocated at predetermined position in the format. As these partspreviously known, it is possible to store the data of these parts instorage devices. The data which are previously known and are comparedwith received data are designated “original data.” The original data maybe composed in a fixed position at a user data area in the predeterminedformat.

Referring to FIG. 16 and FIG. 17, a BER (Bit Error Rate) control unit235 sends a selection signal which designates the first combination ofEMI reducing circuit 223 to a control circuit unit 222 (Step 250 in FIG.17).

Corresponding to the selecting signal, the control circuit unit 222enables the EMI reducing circuits 223 designated by the firstcombination. After enabling the designated EMI reducing circuits 223,the apparatus 220 starts to receive data and the received signals areinput via the RF receiver 228 and the demodulator 229 into acomparing/BER calculating unit 232 as in the form of digital data.

In the comparing/BER-calculating unit 232, a gate (not shown) samplesthe received data in the unit 232 and the part corresponding to theoriginal data is extracted from the received data. Then, data in thepart is compared with the original data read out from a ROM 231 storingthe original data.

BER is calculated in the comparing/BER calculating unit 232 (Step 251 inFIG. 17), and the BER is sent to and stored in a RAM 233 (Step 252 inFIG. 17). On the next step, a BER circuit control unit 235 checkswhether every combination of EMI reducing circuit 223 is selected or not(Step 253, 254, 255 in FIG. 17). The above-mentioned calculation of BERis continued to the end of predetermined combination and every BERcorresponding to each of the combination is stored in the RAM 233.

After the BERs to all combination are stored in the RAM 233, the allBERs are sent to a BER decision unit 234, in which the combinationresulting in a minimum BER is searched (Step 256 in FIG. 17).

The selecting signal indicating the searched combination is sent fromthe BER circuit control unit 235 to the control circuit unit 222 and theEMI reducing circuits 223 are enabled by the control signal from theunit 222.

In the thirteenth preferred embodiment, the switching devices used inEMI reducing circuits 223 are preferably MOS transistors, relays, orbipolar transistors. The reason why these devices are preferable is afaster operation than an operation with other mechanical switchingdevices.

While the selecting process described above can be performed any time,it is preferable to perform at the time of switch-on of the apparatus,the power-supply or at predetermined time-spaced intervals. Particularlythe predetermined time-spaced interval selecting process preventsadversely affecting from the variation of circuit componentscharacteristics by the temperature and aging.

The thirteenth preferred embodiment provides the means for reducing theEMI emitted from the apparatus to the minimum level of the EMI byadjusting the EMI reducing circuits to be enabled.

And still more, the thirteenth preferred embodiment provides the meanscapable of calculating the bit error rate without expensive measuringinstruments so that the apparatus 220 operates in the minimum level ofthe EMI within the limits of the possible.

A fourteenth preferred embodiment of the present invention isschematically shown in FIG. 18. In FIG. 18, the same ones are designatedby similar numerals in FIG. 16. A numeral 300 designates an informationprocessing apparatus having wireless interface. The apparatus 300 has areceiving antenna 301. Signals received by the receiving antenna 301 areinput via a switch 302 to the RF receiver 228. When the switch 226 turnsto “S” as that the switch 302 remains closed, the data stored in the ROM231 are read out and the data are transmitted from the antenna 227 viathe information processing unit 221, the modulator 224, and the RFtransmitter 225. The receiving antenna 301 receives the signals emittedfrom the antenna 227. As the received signals are originally generatedby the data stored in the ROM 231, the data stored in the ROM 231 areused as the original data.

A comparison the received data with the original data, a calculation ofthe error rate, and a selection of the suitable combination of the EMIreducing circuits 223 are done in the same manner as described in thethirteenth preferred embodiment. As the apparatus 300 provided by thethirteenth preferred embodiment has the antenna 227 and the receivingantenna 301, the most suitable combination of EMI reducing circuits 223can be selected even when there is no signal transmitted by otherfacilities or original data are not known. Still more, since the BER ismeasured by the signal transmitted form the apparatus 300 itself, it ispossible to select the most suitable combination of EMI reducingcircuits without long-time work for measurement and expensivemeasurement instruments. The combination of EMI reducing circuits to beenabled is selected so that the combination can give the minimum BERcalculated in the comparing unit 230.

Without the comparing unit 230 or without operating of the comparingunit 230, the information processing apparatus 220 in the thirteenthpreferred embodiment and the information processing apparatus 300 in thefourteenth preferred embodiment can perform to reduce the EMI using theprocedure of selection of the suitable combination of the EMI reducingcircuits as the same manner described in the twelfth preferredembodiment.

A fifteenth preferred embodiment is shown in FIG. 19. In FIG. 19, thesame elements are designated by similar numerals in FIG. 16. The numeral350 refers to a mobile terminal, such as a mobile computer, a notebookcomputer and a laptop computer, which is easy to be carried. The mobileterminal 350 can include a wireless communication device, which ispreferably accommodated in a PCMCIA (Personal Computer Memory CardInternational Association) printed circuit card 351 designated by dotedline in FIG. 19. The antenna 227 is connected to the card 351, ormounted on the card 351.

The card 351 is attached and removed in the PCMCIA standard socket (notshown) provided at the mobile terminal 350. In the card 351 are includedthe modulator 224, the RF transmitter 225, the switch 226, the RFreceiver 228, and the demodulator 229. The information-processing unit221 has PCB(s), which includes CPU (not shown), other circuitry (notshown) for a computer composition, the EMI reducing circuits 223connected to electrical conductive patterns formed on or in the PCB(s),and the control circuit unit 222 which generates signals for selectiveactuation of the switching devices.

The original data included in received signals are compared with theoriginal data stored in the ROM 231. The combination of EMI reducingcircuits 223, which makes the BER minimum, is searched in the comparingunit 230 in the same manner as described in the thirteenth and thefourteenth embodiments shown in FIG. 16 and FIG. 18 respectively. TheEMI reducing circuits 223 in accordance with the most suitablecombination are enabled by closing the switching devices selected by thesignal from the control circuit unit 222. In the fifteenth preferredembodiment, the EMI reducing circuits which are shown in FIG. 4, FIG. 5,FIG. 6, FIG. 7, and FIG. 12 are preferably applied to the EMI reducingcircuits 223. Although the fifteenth preferred embodiment has adetachable wireless communication device, the present invention may beapplied to a mobile terminal having a wireless communication devicearranged on a PCB coupled to the terminal.

The fifteenth preferred embodiment provides the mobile terminal, capableof communicating with a wireless interface, with means for reducing EMI.Using the means, the mobile terminal can communicate with otherfacilities in good condition.

The present invention may be embodied in the other specific formswithout departing the sprit or essential characteristics thereof.

We claim:
 1. A printed circuit board having a printed pattern,comprising: plurality of EMI reducing circuits arranged on said printedcircuit board, each of said EMI reducing circuits including: a circuitelement for decreasing electromagnetic radiation from said printedcircuit board, and a switching device selectively connecting saidcircuit element to a portion of said printed circuit pattern forenabling and disabling said circuit element; a memory device for storinginformation of a plurality of predetermined combination of said EMIreducing circuits to be enabled and disabled; and a control unit fortransmitting signals to said EMI reducing circuits for opening andclosing said switching devices corresponding to said information.
 2. Aprinted circuit board having a printed circuit pattern, comprising: aplurality of EMI reducing circuits arranged on said printed circuitboard, each of said EMI reducing circuits including: a circuit elementfor decreasing electromagnetic radiation from said printed board, and aswitching device selectively connecting said circuit element to aportion of said printed circuit pattern for enabling and disabling saidcircuit element; a wireless communication means; a data storing devicefor storing an original data; a comparing unit for comparing saidoriginal data with a data received by said wireless communication means,and for outputting a data indicating a difference resulting from saidcomparing; and means for deciding said EMI reducing circuits to beenabled in accordance with said data indicating the difference.
 3. Anapparatus including a printed circuit board according to claim 1 furthercomprising: a wireless communication means; a storing device for storingan original data; a comparing unit for comparing said original data witha data received by said wireless communication means, and for outputtinga data indicating a difference resulting from said comparing; and meansfor selecting a data indicating a minimum difference out of a pluralityof said data indicating the difference corresponding to each of saidcombination stored in said memory device, and for transmitting a data ofsaid combination providing in said minimum difference to said controlunit.
 4. The apparatus according to claim 3, wherein said apparatusfurther comprises: a receiver having a receiving antenna, and sendingsignals received by said receiving antenna to said comparing unit; andwherein said wireless communication means emits signals including saidoriginal data; said receiver receives said signals and sends saidsignals to said comparing unit, and said comparing unit compares saidoriginal data with a data in said signal and outputs a data indicating adifference resulting from said comparing of said original data with saiddata.
 5. A mobile terminal including a printed circuit board and adetachable wireless communication module comprising: said printedcircuit board comprising a plurality of EMI reducing circuits, said EMIreducing circuit including a switching device for enabling and disablingsaid EMI reducing circuit; a memory device for storing information of aplurality of predetermined combination of said EMI reducing circuits tobe enabled and disabled; a control unit for transmitting signals foropening and closing said switching devices to said EMI reducing circuitscorresponding to said information; a data storing device for storing anoriginal data; a comparing unit for comparing said original data with adata received by said detachable wireless communication means, andoutputting a data indicating a difference resulting from saidcomparison; and means for selecting a data indicating a minimumdifference out of a plurality of said data indicating the differencecorresponding to each of said combination stored in said memory device,and for transmitting a data of said combination resulting in saidminimum difference to said control unit.
 6. A method for selecting EMIreducing circuits according to claim 1, said method comprising the stepsof: composing a plurality of combination of said EMI reducing circuitsto be enabled and disabled; performing an information processing inenabling said EMI reducing circuits in each of said combination;measuring electromagnetic radiation emitted from said printed circuitboard in performing said information processing; selecting a minimumamount out of amounts of said electromagnetic radiation; deciding acombination of EMI reducing circuits providing said minimum amount; andenabling said combination of said EMI reducing circuits.
 7. The methodaccording to claim 6, wherein said comparing step further comprisessteps of: comparing a datum affected by said electromagnetic radiationwith a true datum of said affected datum; and calculating an amountindicating a difference between said data.
 8. A method for selecting EMIreducing circuits according to claim 5, said method comprising the stepsof: composing a plurality of combination of said EMI reducing circuitsto be enabled and disabled; performing an information processing inenabling said EMI reducing circuits in each of said combination;measuring electromagnetic radiation emitted from said printed circuitboard in performing said information processing; selecting a minimumamount out of amounts of said electromagnetic radiation; deciding acombination of EMI reducing circuits providing said minimum amount; andenabling said combination of said EMI reducing circuits.